1. Field of the invention
This invention relates to an electrode for a secondary cell, a process for its production, and a secondary cell having such an electrode. More particularly, it relates to electrodes for secondary cells as typified by a lithium secondary cell employing lithium in the negative electrode, a lithium secondary cell employing lithium-transition metal in the positive electrode, a nickel-zinc secondary cell or bromine-zinc secondary cell employing zinc in the positive electrode and a nickel-cadmium cell or nickel-hydrogen cell employing nickel hydroxide in the positive electrode, a process for producing such electrodes, and a secondary cell having such electrodes.
2. Related Background Art
In recent years, it is foreseen that the greenhouse effect due to an increase in CO2 in the atmosphere will cause a rise of the earth""s surface temperature. Additional construction of thermal power plants that generate electricity by utilizing energy produced by burning what is called fossil fuels such as petroleum and coal has become difficult because the combustion of such fuels is accompanied by CO2 emissions and because substances other than CO2, such as nitrogen oxides NOx and hydrocarbons, which are said to cause air pollution are released to the atmosphere. In addition, rated operation is preferable in order to control as far as possible the quantity of release of the substances said to cause air pollution. It is also difficult to vary the amount of electricity generation in a short time. Accordingly, under existing circumstances, the electricity is generated so as to be adapted to the daytime, during which power consumption increases, and much of electricity thus generated is wasted without being used. Now, as effective utilization of generated electric power, it is proposed to employ what is called load leveling, which is to store nighttime electric power in secondary cells equipped in homes and so forth so as to level the load.
In the field of electric cars that may discharge no air-pollutive substances when driven, the advent of secondary cells with a long cycle lifetime and a high energy density is long-awaited also in order to provide a substitute for conventional internal combustion engines such as gasoline engines and diesel engines.
The advent of secondary cells with a long cycle lifetime and a high energy density is also long-awaited as power sources of portable machinery such as personal computers, word processors, video cameras and portable telephones.
As compact, light-weight and high-performance secondary cells, JOURNAL OF THE ELECTROCHEMICAL SOCIETY 177, 222 (1970) has reported an example in which a lithium-graphite interlayer compound is applied to the negative electrode of secondary cells. Since then, there has been progress in the development of, for example, what is called xe2x80x9clithium ion cellsxe2x80x9d, which are rocking chair type secondary cells employing carbon as a negative electrode active material, as an interlayer compound incorporated with lithium ions, and as a positive electrode active material where lithium is stored by intercalating it between layers of carbon by the reaction of charging. Some of these cells are being put into practical use. In lithium ion cells, the host material carbon that intercalates lithium between layers as the guest is used in the negative electrode to thereby prevent the dendrite growth of the lithium at the time of charging so that a long lifetime can be achieved in the charging-discharging cycle.
However, in lithium ion storage cells employing carbon as the host material of the negative electrode at which lithium is intercalated, the discharge capacity that can be stably taken out while repeating charging and discharging for a long time is at most the quantity of electricity corresponding to one lithium atom per ten carbon atoms. No cells have been available which can exceed the theoretical capacity of graphite capable of intercalating one lithium atom per six carbon atoms.
Lithium-transition metal oxides in which lithium has been intercalated are also mainly employed as positive electrode active materials of the above lithium ion storage cells. In practice, however, only 40 to 60% of the theoretical capacity is utilized.
Moreover, even in such lithium ion storage cells, lithium may be locally dendrite-deposited on the negative electrode surface when the cells are charged at a great electric current (i.e., charged at a high rate), to cause an internal short, resulting in a lowering of cycle lifetime in some cases.
In storage cells employing zinc in the negative electrode, for example, nickel-zinc storage cells, the zinc may also be dendrite-deposited when charged, which will tend to cause an internal short, providing a cause of obstructing the elongation of cycle lifetime.
The present inventors have presumed that the cause of the problems in the lithium ion storage cells and nickel-zinc storage cells is a low capacity of the collector of an electrode to collect electrons from an active material layer. In order to improve the electron collecting ability of the collector by increasing the surface area of the collector, they have taken note of nickel powder sintered material substrates or foam nickel substrates, employed in collectors of nickel-cadmium storage cells and nickel-hydrogen-occluded alloy storage cells, which are alkaline secondary cells, and have attempted to apply these to the electrodes of the lithium ion storage cells or nickel-zinc storage cells. Here, the nickel powder sintered material substrates are those formed by coating a slurry of a mixture of nickel powder, an organic binder and water on a nickel-plated porous thin steel plate (a core material), followed by sintering, and have an average pore size of 6 to 12 microns and a porosity of 78 to 82%. The foam nickel substrates are those formed by chemically or physically forming a nickel metal coating on the surface of a sheet-like polymeric resin such as urethane foam, having a three-dimensional network structure, and baking the coating to remove the resin, followed by sintering treatment, and have an average pore size of 100 to 300 microns and a porosity of 92 to 96%.
However, as a result of the attempt, both the nickel powder sintered material substrates and the foam nickel substrates were found to have a large thickness. Since the thickness of electrodes can not be made small, the electrode area can not be enlarged in a cell housing having a limited volume. Thus, it was impossible to improve high-rate charging-discharging performance and discharge capacity as expected. Also, because of uneven surface in either of the nickel powder sintered material substrates and the foam nickel substrates, electric fields converged on some places at the time of charging, so that lithium or zinc tended to be dendrite-deposited. Thus, it was impossible to solve the problems involved in the secondary cells utilizing the reaction of lithium ions (hereinafter called lithium secondary cells) and the secondary cells employing zinc in the negative electrode (hereinafter called zinc secondary cells).
The present invention was accomplished taking account of the problems discussed above, and an object thereof is to provide an electrode for a secondary cell, having a long cycle lifetime, a high-energy density and superior performances; a process for its production; and a secondary cell having such an electrode.
Another object of the present invention is to provide an electrode that can prevent or control an increase in impedance of a negative electrode or a positive electrode which may be caused by charging and discharging; a process for its production; and a secondary cell having such an electrode.
Still another object of the present invention is to provide an electrode that can adhere to the active material and can be rapidly charged and discharged; a process for its production; and a secondary cell having such an electrode.
The present invention provides an electrode for a secondary cell, comprising at least a collector that retains an electrochemically active material participating in the cell reaction; wherein the collector comprises a porous metal having micropores with an average diameter not larger than 3 microns (xcexcm).
The present invention also provides a secondary cell comprising at least a first electrode having an active material pertaining to cell reaction and a collector capable of retaining the electrochemically active material, a second electrode provided opposingly to the first electrode via an electrolyte and a separator, and a housing that holds these members; wherein the collector comprises a porous metal having micropores with an average diameter not larger than 3 microns (xcexcm).
The present invention still also provides a process for producing an electrode for a secondary cell, the electrode having a collector capable of retaining an electrochemically active material; which the process comprises the step of reducing an oxidized metallic material to form the collector.